Propellers of this kind may have very different power requirements, according to the operating conditions. With the normal design of drive equipment, the full output of the drive engine, which is usually a supercharged diesel engine, is taken up by the propeller, when the ship is travelling freely. However, if the travel speed of the ship is impeded, for example, if something is being towed by the ship, the rated output of the engine could be greatly exceeded, owing to the propeller characteristics. It is not possible to exceed the rated output of a diesel engine due to the normal limitation of the amount of fuel injected, so that under such a circumstance, the rotational speed of the drive engine is greatly reduced.
With highly supercharged engines, such as those increasingly being used in shipping, operation of the engine at reduced speed is unacceptable, because it greatly reduces the working life of essential components. It causes air starvation and the deposition of carbon, which can cause the piston rings to seize up, for example. The cooling of the engine is also reduced unacceptably.
For this reason, until now, exceeding of the nominal rated output of an engine has been countered by a reduction in the pitch of the propeller blades. By reducing the pitch according to the loading of the drive engine, overloading and engine operation at reduced rotary speed may be avoided. Upon overloading of the drive engine with respect to its normal charging level, appropriate switching contacts were activated, which operated signalling lamps within sight of the operating personnel. When such a signal was given, the pitch of the propeller blades was reduced accordingly until the signal lamps indicated proper engine speed. This type of overloading protection was relatively effective, but it relied too much on the vigilance of an operator and was very inaccurate with regard to regulation.